R/locatePointOnEdge.R

In apear9/SSNdesign: An R Package for Optimal and Adaptive Sampling Designs on Stream Networks

locatePointOnEdge <- function(ssn, rid, ratio){
  # match rid to fid (since lines + coordinates ordered by fid)
  fid <- (0:(nrow(ssn@data) - 1))[which(ssn@data$rid == rid)]
  # get line segments according to criteria
  ssncoords <- ssn@lines[[fid + 1]]@Lines[[1]]@coords
  nsegments <- nrow(ssncoords) - 1
  # get lengths of every segment
  seglengths <- rep(NA, nsegments)
  for(i in 1:nsegments){
    seglengths[i] <- sqrt(sum((ssncoords[i, ] - ssncoords[i + 1, ])^2))
  }
  sumseglengths <- cumsum(seglengths)
  edgelength <- max(sumseglengths)
  sumsegratios <- sumseglengths/edgelength
  # get coord of last vertex before distfromstart
  ids <- 2:(nsegments+1)
  last <- suppressWarnings(max(ids[sumsegratios <= ratio]))
  if(is.infinite(last)){
    last <- 1
  }
  # get coord of last vertex before and after distfromend
  lastvertex <- ssncoords[last, ]
  nextvertex <- ssncoords[last + 1, ]
  # calc as proportion of total edge length
#  lastprop1 <- sumseglengths[last]/edgelength
  loclength <- ratio * edgelength
  if(last - 1 != 0){
    scllength <- (loclength - sumseglengths[last - 1])/seglengths[last]
  } else {
    scllength <- loclength/seglengths[last]
  }
  
  # recalculate new proportion, being the proportion of the enclosed segment along which we will place the sampling point
#  if(!is.infinite(last))
#  newprop <- abs(ratio - lastprop1)/(seglengths[last])
    # calculate coordinate on the constrained edge
  newvertex <- lastvertex + scllength * (nextvertex - lastvertex)
  # return everything.
  return(c(newvertex, loclength))
}